David Allsopp, could you please include me on the "Reply" so that the mail threads make their way to my inbox?

David, are you certain that the explicit_arity attribute is only related to type declarations? I do not believe that to be the case. I understand them to apply to patterns and expressions as well.

(* Declares a variant that accepts a *single* parameter *)

type oneArgTuple = OneArg of (int*int)

(* Works because arity is massaged into whatever is necessary and doesn't need to be expressed at parse time. *)

let OneArg (x,y) = OneArg (1,2)

(* But you can *explicitly* enforce arity and tell the type system that this tuple should be treated as multiple arguments *)

(* That causes this to fail type checking on this tuple pattern! *)

let (OneArg (x,y) [@explicit_arity]) = OneArg (1,2);;

>> Error: The constructor OneArg expects 1 argument(s),

>> but is applied here to 2 argument(s)

(* Similarly, at parse time, you can tell the parser to parse an expression as being multiple arguments *)

let result = OneArg (1,2)[@explicit_arity];;

>> Error: The constructor OneArg expects 1 argument(s),

>> but is applied here to 2 argument(s)

So the fact that you can inform the parser to treat a tuple as multiple arguments to a Constructor, means that the same should be true of polymorphic variants, but it is not. For example, this type checks but it shouldn't:

let (`MyThing (x,y) [@explicit_arity]) = `MyThing (2,2);;

On Fri, Jan 23, 2015 at 12:03 AM, Jacques Garrigue <garrigue@math.nagoya-u.ac.jp> wrote:

The answer is simple: polymorphic variants can only accept one argument
(which may of course be a tuple). The other behavior would have required
a specific syntax for multi-parameter polymorphic variants, since there is
no information associated to the constructor for them.

Jacques Garrigue

On 2015/01/23 15:53, Jordan W wrote:
>
> The OCaml compiler allows distinguishing between variants that accept a single tuple and variant types that accept several parameters. What looks like a variant type accepting a tuple, is actually the later:
>
> type x = TwoSeparateArguments of int * int
> let tuple = (10,10)
> let thisWontWork = TwoSeparateArguments tuple;;
> >> Error: The constructor TwoSeparateArguments expects 2 argument(s), but is applied here to 1 argument(s)
>
> (* Notice the extra parens around the two ints *)
> type x = OneArgumentThatIsATuple of (int * int)
> let thisActuallyWorks = OneArgumentThatIsATuple tuple
>
> The extra parens distinguish at type definition time which of the two is intended.
>
> But OCaml does some automatic massaging of the data that you supply to constructor values.
> let _ = OneArgumentThatIsATuple (4, 5)
> let _ = TwoSeparateArguments (4, 5)
>
> No extra parens are required in this case. But OCaml does give you the ability to annotate patterns and expressions with an "explicit_arity" attribute which allows syntactic distinction between supplying two separate parameters vs. one that happens to be a tuple. This is important for other parser extensions that wish to treat the two distinctly. What OCaml allows (explicit_arity attribute) works well enough.
>
> The only problem is that there doesn't seem to be a way to utilize the same explicit_arity attributes with polymorphic variants. Such attributes are not acknowledged by the type system. Is this intended?
>
> Taking a quick look at typecore.ml, explicit_arity appears to be acknowledged on standard constructors but not polymorphic variants.
> https://github.com/ocaml/ocaml/blob/6e85c2d956c8fd5b45acd70a27586e44bb3a3119/typing/typecore.ml
>
> It seems these should be brought to consistency. I will file a mantis issue unless anyone believes this is intended.
>
> Thank you in advance.
>
> Jordan
>
>